My invention relates to an aqueous acidic bath for electrochemical deposition of a copper coating, especially a shiny and tear-free copper coating, and a coating for reinforcement of conductive pathways on a printed circuit board with outstanding breaking elongation.
A current aqueous acidic bath for electrochemical deposition of a copper coating or layer comprises at least one copper salt, at least one inorganic acid and, if necessary, a compound having a chloride and an amide group-containing compound, an organic sulfur-containing compound with solubilizing group and, if necessary, another organic compound selected from a group of compounds to be defined more precisely in Table III and the following paragraphs.
It has been known for a long time that organic substances may be added to an electrochemical copper bath to attain a shiny coating. However those additives in many cases lead to a deterioration of mechanical properties of the coating, especially hardness and breaking elongation to a passification or surface inertness, which makes necessary a subsequent activation for further treatment. Moreover many inhibitors impair the metal distribution, so that tears arise at passages and edges, especially when the copper layer in a printed circuit with solder is exposed to a thermal treatment.
The numerous compounds already known for this purpose such as thiourea, thiohydantoic acid, thiocarbamic acid ester or thiophosphoric acid ester, have however no practical importance, since the quality of the copper coating obtained with them, especially in regard to the hardness and the breaking elongation, is very poor. Also a combination of any of these compounds with another additive, such as an ethylene oxide addition compound or polyamine, leads to an unsatisfactory result.
A copper bath containing an acid amide of the general formula R--CO--NH.sub.2, in which the R represents an aliphatic or aromatic monomer or a polymeric hydrocarbon group, in combination with an acid containing high-molecular weight compound and an organic sulfur-containing compound with a solubilizing group as described in German Open Patent Application No. 27 46 938 was a significant advance in electrochemical copper bath technology.
This bath has the disadvantage however that it acts optimally only in a very narrow concentration range which frequently does not correspond to what is required in practice. Through-going hole oblateness occurs on exceeding of a few milligrams per liter bath solution so that the circuit boards become torn or fractured during later soldering and are unuseable.
Copper coatings of improved shininess and smoothness made from an aqueous acid copper bath containing at least one copper salt, at least one inorganic acid and, if necessary, an aliphatic hydrocarbon compound having a chloride and an acid amide group, an organic sulfur compound with a solubilizing group and another organic compound selected from a group more precisely defined hereinbelow in Table III are described in U.S. Pat. No. 35 02 551.